1. Field of the Invention
The present invention relates to a circuit for switching the capacitors in a circuit for charging capacitors of an electronic device which is powered by the electric charge produced by solar cells and stored in the capacitors, without having a battery.
2. Description of the Prior Art
FIG. 2 illustrates a conventional capacitor switching circuit. A first resistor R1 and a second resistor R2 are connected to anodes of a first diode D1 and a second diode D2, respectively.
The structure and operation of the capacitor switching circuit will now be described in conjunction with FIGS. 3A, 3B and 3C. The capacitor switching circuit without having the battery is used for an electronic timepiece which has not a battery but a capacitor C1 having a large capacitance, a capacitor C2 having a small capacitance, and a power source or a power source terminal. Hereinafter, the capacitor C1 having the large capacitance is simply referred to as C1, and the capacitor C2 having the small capacitance is simply referred to as C2. Switching elements Tr1 and Tr2 shown in FIGS. 3A, 3B and 3C correspond to the switching elements Tr1 and Tr2 of FIG. 2, and are turned on or off or made conductive or non-conductive by logic or control signals A and B from a logic or control circuit. The power source consists of solar cells, a manually operated generator or an external power source. In the following description, the power source is presumed to be solar cells for easy explanation.
The capacitor switching circuit operates so that the switching elements Tr1 and Tr2 are turned off when the solar cells are irradiated with light and produce an electric charge current that is generated under the condition where the voltages of C1 and C2 are low and the electronic device is not operating. Namely, the current is permitted to flow into C2 only to quickly raise or build up its output voltage, so that the logic circuit is instantly powered and operated and the electronic device exhibits carries out its function. This operation is hereinafter referred to as "quick start" "or build-up", and FIG. 3A represents the condition of quick start. When the output voltage of C2 is still low at the time of quick start and the logic circuit is not operating, the logic signals A and B are not yet stabilized to control the switching elements Tr1 and Tr2. Therefore, the resistors R1 and R2 are connected to the gates or input terminal of the switching elements Tr1 and Tr2 as illustrated in FIG. 2, so that the switching elements are completely turned off.
As the logic circuit detects the fact that the output voltage of C2 is sufficiently raised in excess of a predetermined voltage, the switching element Tr2 is turned on by the logic signal B, whereby the current from the solar cells flows into C1 to electrically charge C1. Hereinafter, this condition is referred to as "C1 charging condition". FIG. 3B represents the C1 charging condition. Under the C1 charging condition, the current from the solar cells flow almost all into C1, and very little current flows into C2. During this time, the logic circuit is supplied with the output voltage of C2. Therefore, the voltage of C2 decreases gradually. As the logic circuit detects that the voltage of C2 beomes lower than a given value, the switching element Tr2 is turned off by the logic signal B, and the quick start condition is assumed. As the quick start condition is assumed, C2 is quickly charged by the solar cells and the C1 charging condition is resumed. Thus the quick start condition represented by FIG. 3A and the C1 charging condition represented by FIG. 3B are alternatingly repeated, so that the voltage of C1 rises gradually.
As the logic circuit detects the fact that the voltage of C1 is raised to a value sufficient for operating the logic circuit, both the switching elements Tr1 and Tr2 are turned on by the logic signals A and B. The condition in which both the switching elements Tr1 and Tr2 are turned on is referred to as "C1 charge completion condition" which is represented by FIG. 3C.
Under the quick start condition and the C1 charging condition, the voltage of C2 drops quickly if the solar cells are no more irradiated with light, and the timepiece ceases operation. Under the C1 charge completion condition, however, the current is supplied from C1 to the logic circuit, and the timepiece continues to work for several days.
In the conventional capacitor switching circuit, however, the electric current flows into the resistors R1 and R2 under the C1 charge completion condition. Since the current is supplied from the capacitors C1 and C2, duration of the timepiece is shortened accordingly.